The invention relates to a process for the production of paraxylene that combines an adsorption stage in a simulated moving bed of a feedstock with eight aromatic carbon atoms and a stage of isomerization in liquid phase of a fraction that is low in paraxylene that is recovered from said bed. It pertains particularly to the synthesis of very pure paraxylene for producing an intermediate petrochemical, terephthalic acid.
The composition of the aromatic feedstocks with eight carbon atoms varies extensively according to their origin. Generally, the content of para-and orthoxylene isomers is close to 50%, so that a single process does not make it possible to maximize the production of paraxylene. It is then necessary to combine an adsorption stage of the feedstock in a zeolitic sieve that delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in orthoxylene and metaxylene followed by an isomerization stage of this fraction that is low in paraxylene, as is described in the patent (G.B. 1420796). During this isomerization stage, the ratio of isomers at equilibrium is reestablished since the desirable isomers are produced at the expense of undesirable isomers.
Now, taking into account the variety of products that are introduced into the isomerization zone, the conditions of the isomerization reaction cannot be optimized. There generally follow secondary reactions of dismutation of ethylbenzene that lead to the formation of benzene and heavy aromatic hydrocarbons and dismutation of xylenes that are transformed into toluene and heavy aromatic compounds, which complicates the downstream separations and which reduces the amount of desirable isomers for the production of p-xylene and therefore the final product yield.
A prior written description in patent application FR 2 768 724, describes a combination of stages for isomerization in liquid phase of a fraction that is high in metaxylene and orthoxylene and for isomerization in vapor phase of a fraction that is high in ethylbenzene. Whereby the isomerization conditions are not adequately optimized, there also results the formation of secondary products that interfere downstream with the separation of isomers by adsorption.
In addition, a pelletized TPZ-3 catalyst that is used for the vapor phase isomerization of a feedstock that consists of ethylbenzene or a feedstock that consists of xylenes is known by patent application EP-A-51318. This application, however, ignores the incidence of secondary products in a scheme of processes that should result in the optimized production of very pure paraxylene and disregards the influence of the shaping of the catalyst.
One of the objects of the invention is to eliminate the drawbacks of the prior art and therefore to optimize the isomerization reactions of the isomers of xylenes, and thus to reduce the impurities and to increase the yield of p-xylene produced.
Another object is to combine an adsorption stage that uses in particular the toluene as a desorbent with an isomerization stage of the xylenes that use toluene as a diluent and a separated stage for isomerization of the ethylbenzene.
Another object is to isomerize separately the ethylbenzene that was previously separated with a suitable catalyst under a judiciously selected shaping.
It was noted that by combining an adsorption stage in a simulated moving bed and a catalytic isomerization stage of the collected fraction that is low in paraxylene and that contains a substantial amount of toluene and therefore in liquid phase and with no hydrogen, good results and a simplified use were observed. In addition, a substantial savings of distillation equipment was made. More specifically, the invention relates to a process for the production of paraxylene from a feedstock of aromatic hydrocarbons with eight carbon atoms that comprises orthoxylene, metaxylene, paraxylene and ethylbenzene, in which hydrocarbon feedstock (1) is enriched with ethylbenzene in an enrichment zone (2), a first fraction (3) that for the most part contains ethylenebenzene is recovered, the first fraction is isomerized in a catalytic isomerization zone (40) in vapor phase in the presence of hydrogen with a catalyst, an isomerate is recovered, the isomerate is distilled in a so-called stabilization column (5) to eliminate its light fractions, and residual isomerate (8) of the stabilization column is recycled in enrichment zone (2), whereby enrichment zone (2) delivers a second fraction (4) that is distilled in a second distillation column (9), a distillate (10) that contains the orthoxylene, metaxylene, paraxylene and a minimum quantity of ethylbenzene is recovered, said distillate (10) is recycled in at least one adsorption column, an adsorption stage in a simulated moving bed of a feedstock that comprises said distillate (10) is produced in the adsorption column that contains a zeolitic sieve, in the presence of a desorbent, a first fraction that is high in paraxylene and a second fraction that is low in paraxylene and that contains desorbent, metaxylene, orthoxylene and ethylbenzene in a quantity at most equal to 15% by weight beyond the desorbent are recovered, and one or the other of the following sequences are produced:
either said second fraction is isomerized in liquid phase in another catalytic isomerization zone (26), the isomerate is distilled in a distillation column (27), and an isomerate (30) from which desorbent has essentially been removed is recovered,
or the second fraction is distilled in a distillation column (27), a fraction that contains metaxylene and an adequate quantity of desorbent are laterally drawn off (line 45), said fraction drawn-off laterally in liquid phase into another catalytic isomerization zone (26) is isomerized at least in part, the isomerized fraction is introduced (line 37) into same distillation column (27) below the lateral draw-off point of said column, optionally a portion (47a) of the fraction that is drawn off between the lateral draw-off point and the point of introduction of the isomerized fraction is recycled to carry out a washing, and an isomerate (30) from which the desorbent is removed is recovered, and isomerate (30) from which desorbent is removed is recycled in the adsorption column,
the process is characterized in that the catalyst of the isomerization zone in vapor phase comprises an EUO-structural-type zeolite.
The EUO-structural-type zeolite that is contained in the catalyst, in particular the EU1 zeolite, the ZSM50 zeolite or the TPZ-3 zeolite and their process of production are described in the literature, for example patent EP-B-42226, U.S Pat. No. 4,640,829 or EP-A-51318, and are incorporated as references in patent application EP-A-923 987.
The catalyst in ball form or extrudate form can contain:
from 1 to 90%, preferably 3 to 60% by weight of at least one EUO-structural-type zeolite that comprises silicon and at least one T element that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum and boron, whose Si/T atomic ratio is greater than 5, advantageously between 5 and 100, inclusive, preferably between 5 and 80, inclusive, and also preferably between 5 and 60, inclusive. Said zeolite is at least in part in acid form, i.e., in hydrogen form (H+), whereby the sodium content is such that the Na/T atomic ratio is less than 0.5, preferably less than 0.1 and even more preferably less than 0.02,
from 0.01 to 2%, inclusive, and preferably from 0.05 to 1.0%, inclusive, by weight, of at least one metal of group VIII of the periodic table, preferably selected from the group that is formed by platinum and palladium and even more preferably platinum, whereby said metal of group VIII is deposited on the zeolite or on the binder, preferably selectively on the binder and that has a dispersion that is measured by, for example, chemisorption, for example by H2xe2x80x94O2 titration or by, for example, chemisorption of carbon monoxide, between 50 and 100%, inclusive, preferably between 60 and 100%, inclusive, and still more preferably between 70 and 100%, inclusive. In addition, the macroscopic distribution coefficient of said metal(s), obtained from its profile that is determined by Castaing microprobe, whereby said distribution coefficient is defined as the ratio of the concentrations of said metal in the core of the grain relative to the edge of the same grain, is between 0.7 and 1.3, inclusive, preferably between 0.8 and 1.2, inclusive,
optionally from 0.01 to 2%, inclusive, and preferably between 0.05 and 1.0%, inclusive, by weight, of at least one metal of the group that is formed by groups IIIA and IVA of the periodic table, preferably selected from the group that is formed by tin and indium,
optionally sulfur whose content is such that the ratio of the number of sulfur atoms to the number of metal atoms of group VIII that are deposited is between 0.5 and 2, inclusive,
the addition to 100% by weight of at least one binder, preferably alumina.
The catalyst may have a mechanical resistance such that the crushing value in the bed is greater than 0.7 MPa (Shell method).
The toluene can be used as a desorbent in the adsorption process in a simulated moving bed. It can thus be the diluent that is required for isomerization in liquid phase of the fraction that is obtained from the simulated moving bed that essentially contains the orthoxylene and metaxylene, and the toluene with a limited quantity of ethylbenzene.
According to a characteristic of the invention, the ethylbenzene content of the second fraction that is low in paraxylene can reach, outside of desorbent, at most 10% by weight and preferably 5 to 8% by weight.
It is possible to draw off from the distillation column that treats the isomerate or that treats the first fraction that is high in paraxylene a fraction that consists essentially of the desorbent that is recycled at least in part in the adsorption column.
The liquid phase isomerization can be carried out under the following conditions:
Temperature lower than 300xc2x0 C., preferably between 200 and 260xc2x0 C.,
Pressure lower than 40 bar, preferably between 20 and 30 bar,
Desorbent/isomerization feedstock ratio: less than 15%, preferably 10 to 12% by weight,
Zeolitic catalyst: ZSM5, for example,
Volumetric flow rate (V.V.H.) less than 10 hxe2x88x921, preferably between 2 and 4 hxe2x88x921. 
By thus operating in liquid phase that is preferably diluted with toluene, at low temperature on any catalyst that can isomerize the hydrocarbons in liquid phase, the conversion into paraxylene is promoted, and the dismutation reactions of the ethylbenzene and xylenes that lead to the formation of heavy hydrocarbons are avoided.
The feedstock of aromatic hydrocarbons, low in ethylbenzene, that is introduced into the adsorption zone in a simulated moving bed can be obtained from said enrichment zone, which is a distillation of a hydrocarbon mixture or a zone for adsorption of this mixture on a specific adsorbent bed.
According to a first variant, said feedstock comprises a residue of the first distillation column, into which was introduced the mixture of ethylbenzene, metaxylene, paraxylene and orthoxylene and which is regulated such that at least 75% by weight of the ethylbenzene is recovered as distillate.
This residue can be introduced into a second distillation column, and a distillate that contains orthoxylene, metaxylene and paraxylene is drawn off that is sent into the adsorption column, and a second residue that contains heavy C9+hydrocarbons.
It is possible to operate the second distillation column such that it delivers said residue that contains orthoxylene and the heaviest hydrocarbons; said residue is distilled in a fourth so-called rerun distillation column (12); and a distillate that contains orthoxylene that is recycled in the isomerization zone in liquid phase is drawn off.
The distillate of the first distillation column that contains ethylbenzene is isomerized in a catalytic isomerization zone in vapor phase in the presence of hydrogen, and the second isomerate that is obtained is distilled in a so-called stabilization column to eliminate its light fractions then recycled in the first distillation column.
The conversion into paraxylene is thus maximized.
The vapor phase isomerization in the isomerization zone can be carried out under the following conditions:
Temperature higher than 300xc2x0 C., preferably 350 to 480xc2x0 C.,
Pressure lower than 40 bar, preferably 5 to 20 bar,
Hourly volumetric flow rate: less than 10 hxe2x88x921, preferably between 0.5 and 6 hxe2x88x921,
Catalyst that contains an EUO-structural-type zeolite,
H2/hydrocarbon ratio that is less than 10, preferably between 3 and 6.
Since the xylenes are absent from the isomerization feedstock, the size of the isomerization unit is small, and the conversion per pass of ethylbenzene is high. Therefore, the recycling rate is lower, the overall feedstock volume is lower, and the catalyst volume is reduced.
Relative to a conventional isomerization in vapor phase of the entire fraction that is low in paraxylene that would comprise a mixture of ethylbenzene and xylenes, the hydrogen recycling will be small, whereby all of these advantages result in substantial savings.
All of the catalysts that are able to isomerize the hydrocarbons with eight carbon atoms are suitable for this invention. A catalyst that contains an EUO-structural-type zeolite and at least one metal of group VIII of the periodic table (Handbook of Chemistry and Physics, 45th Edition, 1964-1965) are preferably used in a ratio by weight that is described above. The EU-1 zeolite and the platinum are preferably used as a metal of group VIII.
According to this variant, the ethylbenzene-enriched fraction is isomerized under optimal conditions, and the quantity of hydrogen introduced is consequently adjusted, and it is immaterial that a minimum quantity of xylenes is present in the isomerization feedstock. The consumption of hydrogen is consequently reduced to the minimum. The use of a catalyst that contains an EUO-structural-type zeolite makes it possible to reduce significantly the parasitic secondary reactions of dismutation, transalkylation and cracking which result in the formation of benzene, toluene, heavy hydrocarbons and paraffins and therefore in improving the overall yield per paraxylene pass.
Furthermore, all of the isomerizations, one at low temperature and in toluene liquid phase that works on the xylenes, the other at high temperature in vapor phase that works specifically on ethylbenzene, are easier to use and more selective.
Thus, everything works toward a greater purity and a higher yield of paraxylene.
The effluent that is obtained, after having been introduced into a stabilization column to remove light gases from it, is separated by distillation into a distillate that contains benzene and into a residue that comprises heavier hydrocarbons that are also produced by dismutation, which can be recycled into the first distillation column that receives the feedstock.
According to a second variant of the process that also promotes the production of pure ethylbenzene, the adsorption feedstock in a simulated moving bed comprises a fraction that is low in ethylbenzene and that can result from a specific adsorption of a mixture of ethylbenzene, metaxylene, paraxylene and orthoxylene on a specific adsorbent in the presence of an adequate desorbent, suitable for separating said fraction from another fraction that contains at least the majority of the ethylbenzene and preferably approximately all of the ethylbenzene.
The adsorption of the mixture to recover at least the majority of the ethylbenzene can be carried out in a simulated moving bed, preferably at simulated countercurrent, in the presence of a zeolitic adsorbent that contains at least one element that is selected from the group of elements K, Rb, Cs, Ba, Ca and Sr, and optionally water. The conditions of this particular adsorption are described in, for example, U.S Pat. Nos. 5,453,560, 4,613,725, 4,108,915, 4,079,094 and 3,943,182.
The operating conditions of the first distillation column or those of the specific adsorption of the hydrocarbon mixture for recovering at least the majority of the ethylbenzene will in general be such that a fraction is recovered that contains at least 85% by weight of ethylbenzene and preferably at least 90% by weight, which will then be isomerized in vapor phase in the presence of hydrogen to maximize the production of paraxylene.